1. Technical Field
The present disclosure relates to combustion chambers in direct-injection diesel engines.
2. Background Art
Because of the combustion nature of a diesel engine (compression ignition) cold start and cold operation performance are the hardest requirements for a diesel engine. These requirements drive diesel engines to use cold start aids, one such aid is having the combustion chambers of diesel engines equipped with glow plugs to aid cold start performance, thereby minimizing misfire and white smoke (unburned fuel) during operation directly after a cold start. Optimum compression ratios for best efficiency in real world engines occur at a compression ratio of 15 to 1. To further help improve this cold start performance and operation; historically compression ratios of diesel engines were raised to levels much higher than the optimum of 15 to 1 to increase compression temperatures in the cylinder during cold operation. With the advent of lower emissions requirements for engines, one enabler to help engines to meet those lower requirements (particularly NOx) is to lower compression ratios. This increases the importance of glow plugs in diesel engines and requires them to run at hotter temperatures for longer times further emphasizing the importance of placement in relation to the fuel spray in the combustion chamber.
Combustion chambers of diesel engines are equipped with glow plugs to aid cold start performance. A glow plug is typically mounted in the cylinder head with a tip of the glow plug placed tangentially along the outside envelope of a fuel injector spray plume. When a current is provided to the glow plug, the temperature of the glow plug tip heats up and serves as a hot spot or ignition site for fuel vapor which comes in contact with, or in the vicinity of, the tip. Once some of the fuel is ignited, the energy release from the combusting fuel causes a pressure and temperature rise in the cylinder charge in the combustion chamber and aiding in auto-ignition of the remaining fuel throughout the chamber.
Various factors are considered in determining where to place the glow plug in the combustion chamber. The location should be one with the potential for no, or very few, liquid fuel droplets to impact the glow plug tip anytime the engine is operating to ensure full operational life of the glow plug. The glow plug placement also includes operation when the glow plug is not on, to minimize impact on air flow in the vicinity of the glow plug. Also, the glow plug tip is placed in a region of the combustion chamber where the air/fuel ratio is in an appropriate range so that the fuel vapor readily ignites. And, of course, the glow plug is mounted in the cylinder head in a region which is not otherwise occupied by an intake port, an exhaust port, an injector, a sensor, or other component.
A swirl flow is typically induced in the combustion chamber gases to promote mixing and air utilization. The swirl can cause the fuel jets to be displaced from a straight line trajectory. The amount of displacement depends on the operating condition, air flow into the chamber, the fuel injection pressure, i.e., momentum of the fuel spray, and the distance that the spray has traveled from the injector. As mentioned above, it is important to place the glow plug in a location where a suitable air and fuel concentration exists. In the prior art, the glow plug is placed close to the injector nozzle one the side of the fuel spray, at that small radius, the swirl has less influence on the fuel spray location, thus more certainty that the fuel vapor is in contact with the glow plug shortly after an injection commences.
In engines with 4 valves per cylinder, the maximum valve size is partially dictated by having sufficient material between adjacent valves to provide the necessary strength to withstand combustion pressures. In prior art on engines with 4-valve heads, the glow plug is placed near the injector in the valve bridge area (necked region in between adjacent valves), which compromises the strength of the cylinder head. Consequently, the valve sizes are reduced to allow a larger bridge thickness to accommodate the glow plug or the peak combustion pressure that the combustion chamber can withstand is reduced, either of which lowers the maximum power that the engine can develop.